The present invention pertains to radio signal receivers and more particular a global positioning system (GPS) digital receiver.
The global positioning system is a constellation of satellites which transmits navigation information via radio signals. Time and position may be calculated by receivers which are able to receive and process these radio signals. The satellites of the GPS constellation broadcast two BPSK modulated signals at L-band, 1575.42 megahertz (L1) and 1227.6 megahertz (L2). The modulated signals include psuedorandom noise codes and data. The L1 signal carrier is modulated in quadrature with both a clear acquisition code (CA code) and a precise code (P code). The chipping rate of the CA code is 1.023 megahertz and the P code is 10.23 megahertz. The L2 frequency is modulated with only one code, normally the P code.
It is necessary to track at least four satellites of the GPS constellation in order to compute a GPS receiver's position in three dimensions and determine time. One method of tracking at least four satellites, is to assign one receiver channel to track each of the satellites. This is termed continuous tracking. Another method of tracking four satellites with fewer than four channels requires time-sharing of the channels among the four or more satellite signals. This type of tracking is referred to as sequencing.
Data loss may result from the time required to lockup or synchronize to a particular satellite signal during the sequencing process. Accordingly, such operation requires circuitry which may be set up rapidly by the control circuitry when the time-sharing operation causes switching from one satellite to another. In addition for digital receivers, high Doppler frequencies require high-speed logic to rapidly process the digitized signals.
A receiver whose architecture and design is flexible to the extent of being able to expand the number of channels with a minimum of additional components and operate in either the continuous or sequencing modes will apply to the largest number of applications resulting in the lowest cost. The prior art GPS receivers do not provide this flexibility and hence low-cost features. Another short coming of the prior art is the inability to provide a low-cost integrated circuit chip set, including oscillator, which embodies the circuitry of requisite speed to handle a number of channels for sequencing or continuous tracking mode operations. Typically such circuitry requires several integrated circuits and discrete components which in some implementations must be matched for signal delay and phase shift.
Accordingly, it is an object of the present invention to provide a multi-channel GPS digital receiver which operates in the continuous or sequencing modes at higher user velocities while maintaining circuitry simplicity, accuracy, and is low in cost.